Semiconductor circuits are fabricated on chips th at are then mounted in various forms of packages that are, in turn, incorporated into larger circuits that are assembled on printed circuit boards or the like. Ideally, the packaging operation is carried out on a large number of dies that are packaged together. In this arrangement, the dies are connected to a lead frame that consists of a number of copper traces that are connected together to form a single fabric. After the dies are bonded to the traces, connections are made between the pads on the die and the traces via wire bonds, or the like. A plastic layer is molded over the dies to protect the dies. The resultant sheet is then divided into individual packaged circuits by cutting the sheet along lines that separate the various dies and remove the portions of the fabric that were intended merely to keep the sheet together during the packaging process.
One class of integrated circuit requires that the space over the die be a cavity that is evacuated or filled with a gas. For example, RF circuits require such an air gap. This cavity must be sealed to protect the integrated circuit and or prevent the environment over the die from being altered by the air that surrounds the package.
A number of prior art methods have been proposed to provide such a cavity in an inexpensive plastic package. For example, U.S. Pat. No. 5,946,556 teaches a packaging system in which a lid is used to create a cavity over the die. This design requires that the air cavity be sealed using adhesive. In addition, the lid covers must be attached to the dies one at a time. The adhesive seal of the lid to the underlying portion of the package is not sufficiently reliable. In addition, the need to individually place the lids over the dies substantially increases the manufacturing time, and hence, the cost of the packaged part. An arrangement which utilizes the mold compound to seal the lid over the air cavity is taught in U.S. Pat. No. 5,362,679. While this arrangement solves the problem associated with the lack of reliability of the adhesive seats, the method still requires that each of the metal caps be individually placed over the dies prior to the encapsulation process. Hence, the problems associated with manufacturing time still remain.